Defect Engineering in Bi-Based Photo/Electrocatalysts for Nitrogen Reduction to Ammonia

Main group Bi-based materials have gained popularity as N2 reduction reaction (NRR) photo/electrocatalysts due to their ability to inhibit competitive H2 evolution reactions (HER) and the unique N2 adsorption activities. The introduction of defects in Bi-based catalysts represents a highly effective strategy for enhancing light absorption, promoting efficient separation of photogenerated carriers, optimizing the activity of free radicals, regulating electronic structure, and improving catalytic performance. In this review, we outline the various applications of state of the defect engineering in Bi-based catalysts and elucidate the impact of vacancies on NRR performance. In particular, the types of defects, methods of defects tailoring, advanced characterization techniques, as well as the Bi-based catalysts with abundant defects and their corresponding catalytic behavior in NRR were elucidated in detail. Finally, the main challenges and opportunities for future development of defective Bi-based NRR catalysts are discussed, which provides a comprehensive theoretical guidance for this field.

Constructing High-Active Surface of Plasmonic Tungsten Oxide for Photocatalytic Alcohol Dehydration

Plasmonic semiconductors with broad spectral response hold significant promise for sustainable solar energy utilization. However, the surface inertness limits the photocatalytic activity. Herein, a novel approach is proposed to improve the body crystallinity and increase the surface oxygen vacancies of plasmonic tungsten oxide by the combination of hydrochloric acid (HCl) regulation and light irradiation, which can promote the adsorption of tert-butyl alcohol (TBA) on plasmonic tungsten oxide and overcome the hindrance of the surface depletion layer in photocatalytic alcohol dehydration. Additionally, this process can concentrate electrons for strong plasmonic electron oscillation on the near surface, facilitating rapid electron transfer within the adsorbed TBA molecules for C-O bond cleavage. As a result, the activation barrier for TBA dehydration is significantly reduced by 93% to 6.0 kJ mol-1, much lower than that of thermocatalysis (91 kJ mol-1). Therefore, an optimal isobutylene generation rate of 1.8 mol g-1 h-1 (selectivity of 99.9%) is achieved. A small flow reaction system is further constructed, which shows an isobutylene generation rate of 12 mmol h-1 under natural sunlight irradiation. This work highlights the potential of plasmonic semiconductors for efficient photocatalytic alcohol dehydration, thereby promoting the sustainable utilization of solar energy.

Polyoxometalates tailoring of frustrated Lewis pairs on Ce-doped Bi2O3 for boosting photocatalytic CO2 reduction

Constructing frustrated Lewis pairs (FLPs) on catalysts will provide catalytic sites to activate CO2 and boost photocatalytic CO2 reduction. Herein, a Ce-doped bismuth oxide (CeBiOX) with FLPs was designed by loading [(α-SbW9O33)2Cu3(H2O)3]12- (Cu3) via strong electrostatic interactions to create oxygen vacancies (OVs). Detailed experiments and measurements showed that Cu3 could regulate the FLPs and optimize the band structure of CeBiOX to boost photocatalytic CO2 reduction. In particular, the Cu3/CeBiOX composite exhibited the highest yields of CO (42.85 μmoL g-1) and CH4 (13.23 μmoL g-1), being 6.6 and 3.3 times, and 4.9 and 6.3 times higher than those of pristine Bi2O3 and CeBiOX, respectively. This work provides a significant and mild approach to obtaining advanced catalysts with tuneable FLPs for more fields.

Isolated Ni Atoms Enable Near-Unity CH4 Selectivity for Photothermal CO2 Hydrogenation

Photothermal hydrogenation of carbon dioxide (CO2) into value-added products is an ideal solution for addressing the energy crisis and mitigating CO2 emissions. However, achieving high product selectivity remains challenging due to the simultaneous occurrence of numerous competing intermediate reactions during CO2 hydrogenation. We present a novel approach featuring isolated single-atom nickel (Ni) anchored onto indium oxide (In2O3) nanocrystals, serving as an effective photothermal catalyst for CO2 hydrogenation into methane (CH4) with a remarkable near-unity (∼99%) selectivity. Experiments and theoretical simulations have confirmed that isolated Ni sites on the In2O3 surface can effectively stabilize the intermediate products of the CO2 hydrogenation reaction and reduce the transition state energy barrier, thereby changing the reaction path to achieve ultrahigh selective methanation. This study provides comprehensive insights into the design of single-atom catalysts for the highly selective photothermal catalytic hydrogenation of CO2 to methane.

Flower-like Ag-decked non-stoichiometric Bi2O3-x/rGO hybrid nanocomposite SERS substrates for an effective detection of Rhodamine 6G dye molecules

In early years, SERS-active substrates were generally noble metals. However, their practical applications were limited due to their poor biocompatibility, low uniformity and high cost. Recently, the utilization of semiconductor SERS-active substrates has greatly expanded the applications of SERS in many fields. However, metal-free SERS-active substrates have a low enhancement factor (EF), which can be overcome by adjusting their oxygen deficiency or through the effective preparation of non-stoichiometric semiconducting oxide materials. This is the key strategy and may work as an efficient and simple way to achieve high sensitivity and obtain an enhancement factor (G-factor) comparable to that of noble metals. Here, we report the preparation of flower-like rGO-Bi2O3/Bi2O2.75 and rGO-Ag-Bi2O3/Bi2O2.75 hybrid thin film nanocomposites using a liquid/liquid interface method (LLI) for the first time. In addition to the synergic effect of different enhancement mechanisms, the 3-D flower-like morphology of the substrate shows more favourable properties to improve the G-factor due to the existence of more hotspots. The rGO-Ag-Bi2O3/Bi2O2.75 hybrid thin-film nanocomposites show an EF of 1.8 × 109 with a detection ability of up to 1 nM towards Rhodamine 6G (R6G), which is highly toxic to humans and the aquatic environment.

Heterophase junction engineering: Enhanced photo-thermal synergistic catalytic performance of CO2 reduction over 1T/2H-MoS2

Photocatalytic CO2 reduction technology has been proposed as a promising solution to the greenhouse effect and energy crisis. However, the lower quantum efficiency limits its practical applications. Here, we have significantly improved the photocatalytic CO2 reduction performance of MoS2 by coupling the heterophase junction (1T/2H-MoS2) construction and photo-thermal synergy strategies. At 200 °C and 42 mW·cm-2 of 420 nm LED irradiation, the CO production rate of 1T/2H-MoS2 reached 35.3 μmol·g-1·h-1, which was 3.5 and 2.8 times that of 1T-MoS2 and 2H-MoS2, respectively. In addition, only faint CO was detected under sole photo- or sole thermal catalysis conditions. Mechanism studies showed that COOH* was the key intermediate in the photo-thermal synergistic catalytic CO2 reduction over 1T/2H-MoS2. The heterophase junction engineering significantly facilitated the separation of photogenerated carriers, and the introduction of heat accelerated the charge migration and surface reaction rates. Our work provides innovative insights into the catalyst design and mechanism studies for photo-thermal synergistic catalytic CO2 reduction.

Synergistic Enhancement of Photocatalytic CO2 Reduction by Built-in Electric Field/Piezoelectric Effect and Surface Plasmon Resonance via PVDF/CdS/Ag Heterostructure

Photocatalytic reduction of CO2 using solar energy is an effective means to achieve carbon neutrality. However, the photocatalytic efficiency still requires improvements. In this study, polyvinylidene fluoride (PVDF) ferroelectric/piezoelectric nanofiber membranes are prepared by electrospinning. Cadmium sulfide (CdS) nanosheets are assembled in situ on the surface of PVDF based on coordination between F- and Cd2+ , and then Ag nanoparticles are deposited on CdS. Because of the synergistic effect between localized surface plasmon resonance of Ag nanoparticles and the built-in electric field of PVDF, the CO2 photocatalytic reduction efficiency using PVDF/CdS/Ag under visible light irradiation is significantly higher than that of any combination of CdS, CdS/Ag, or PVDF/CdS. Under micro-vibration to simulate air flow, the CO2 reduction efficiency of PVDF/CdS/Ag is three times higher than that under static conditions, reaching 240.4 µmol g-1 h-1 . The piezoelectric effect caused by micro-vibrations helps prevent the built-in electric field from becoming saturated with carriers and provides a continuous driving force for carrier separation.

Ultrathin CuBi2O4 on a bipolar Bi2O3 nano-scaffold: a self-powered broadband photoelectrochemical photodetector with improved responsivity and response speed

CuBi₂O₄ is a promising photoactive material for photoelectrochemical (PEC) broadband photodetectors due to its suitable band structure, but its photo-responsivity is severely limited by the short carrier diffusion length and long light penetration depth. To address the trade-off between light absorption and charge separation, a nano-structured bipolar Bi₂O₃ host scaffold was coupled with an ultrathin CuBi₂O₄ light absorbing layer to construct a host-guest Bi₂O₃/CuBi₂O₄ photocathode. The work function of the bipolar Bi₂O₃ scaffold lies in between FTO and CuBi₂O₄, making Bi₂O₃ a suitable back contact layer for hole transport. Compared with the flat CuBi₂O₄ and Bi₂O₃ scaffold counterpart, the nanostructured Bi₂O₃/CuBi₂O₄ exhibits significantly improved light absorption and enhanced charge separation efficiency. The Bi₂O₃/CuBi₂O₄ PEC photodetector can be self-powered and demonstrates a broad photo-response ranging from ultraviolet (UV) to near infrared (NIR). It shows a high responsivity of 75 mA W^-1 and a remarkable short response time of 0.18 ms/0.19 ms. Bi₂O₃/CuBi₂O₄ prepared by magnetron sputtering demonstrates great potential for rapid PEC photodetection in a wide optical domain.

Understanding Wavelength-Dependent Synergies between Morphology and Photonic Design in TiO2-Based Solar Powered Redox Cells

Solar powered redox cells (SPRCs) are promising for large-scale and long-term storage of solar-energy, particularly when coupled with redox flow batteries (RFBs). While efforts have primarily focused on heterostructure engineering, the potential of synergistic morphology and photonic design has not been carefully studied. Here, we investigate the wavelength-dependent effects of light-absorption and charge transfer characteristics on the performance of gold decorated TiO₂-based SPRC photoanodes operating with RFB-compatible redox couples. Through an in-depth optical and photoelectrochemical characterization of three complementary TiO₂ microstructures, namely nanotubes, honeycombs, and nanoparticles, we elucidate the combined effects of nanometer-scale semiconductor morphology and plasmonic design across the visible spectrum. In particular, thin-walled TiO₂ nanotubes exhibit a ∼ 50% increase in solar-to-chemical efficiency (STC) compared to thick-walled TiO₂ honeycombs thanks to improved charge transfer. Au nanoparticles both increase generation and interfacial charge transfer (above bandgap) and promote hot carrier injection (below bandgap) leading to a further 25% increase in STC. Overall, Au/TiO₂ nanotubes achieve a high photocurrent at 0.098 mA/cm² and an excellent STC of 0.06%, among the highest with respect to the theoretical limit. The incident photon to current efficiency and internal quantum efficiency are also superior to those of bare TiO₂ showing maximum values of 54.7% and 67%, respectively. Overall, nanophotonic engineering that synergistically combines morphology optimization and plasmonic sensitization schemes offer new avenues for improving rechargeable solar-energy technologies such as solar redox flow batteries.

A novel TiO2-x/TiN@ACB composite for synchronous photocatalytic Cr(VI) reduction and water photothermal evaporation under visible/infrared light illumination

Relatively large band-gap, fast charge carriers recombination, and mono-functionality of photocatalytic materials are still representing stumbling hurdles against their optimal usage for water cleaning. Herein, a novel black titanium oxide/plasmonic titanium nitride@activated coconut biochar (TiO2-x/TiN@ACB) composite was designed to have both photocatalytic and photothermal functions. Intermediate states of black TiO2-x, plasmonic effect of TiN, and high electrons (e-) capacity of biochar enhanced band-gap narrowing, light absorbance extension, and charge carriers separation respectively. Black TiO2-x and plasmonic TiN sensitization via visible/infrared (Vis/IR) portion of photonic spectrum in addition to the confirmed close contact of composite constituents explained the demonstrated major role of e- in photocatalytic mechanism through efficient excitation and facile transfer. Thanks to black photocatalytic semiconductor and carbonic materials for their ultimate photons harnessing and efficient photothermal conversion where the composite exhibited a remarkable photothermal water evaporation upon Vis/IR illumination as well. TiO2-x/TiN@ACB composite revealed 92.8 and 89.7% photocatalytic reduction of hexavalent chromium (Cr(VI)) and water evaporation efficiencies up to 92.9 and 51.1% upon IR and Vis light illumination respectively. This study proposes a new approach for efficient water cleaning by coupling of oxygen deficient and plasmonic semiconductors supported on naturally derived carbonic material as a broad spectrum harvester and bi-functional photocatalytic and photothermal material.

Modulating CoFeOX Nanosheets Towards Enhanced CO2 Photoreduction to Syngas: Effect of Calcination Temperature and Mixed-Valence Multi-Metals

CoFeO_X nanosheets were synthesized by a facile coprecipitation and calcination method. The effect of calcination temperature on the crystal texture, morphology and surface areas of CoFeO_X were fully explored. CoFeO_X sample calcined at 600 °C (CoFeO_X -600) showed superior catalytic performance for the reduction of CO₂ under visible light. Compared with the pure Ru(bpy)₃ ²⁺ -sensitized CO₂ reduction system, the CoFeO_X -added system achieved 19-fold enhancement of CO production (45.7 μmol/h). The mixed valence state and nanosheet-like structure of CoFeO_X cocatalyst support its ultra-high charge transfer and abundant CO₂ active adsorption sites exposure, which promote the separation of photogenerated charges, and thus improve the photocatalytic CO₂ reduction activity. Carbon source of CO from CO₂ was verified by ¹³ CO₂ isotopic labelling experiment. Repeated activity experiments confirmed the good stability of CoFeO_X in the CO₂ photoreduction system. This work would provide prospective insights into developing novel cost-effective, efficient, and durable non-precious metal cocatalysts to improve the efficiency of photocatalytic reduction of CO₂ .

Full-spectrum nonmetallic plasmonic carriers for efficient isopropanol dehydration

Plasmonic hot carriers have the advantage of focusing, amplifying, and manipulating optical signals via electron oscillations which offers a feasible pathway to influence catalytic reactions. However, the contribution of nonmetallic hot carriers and thermal effects on the overall reactions are still unclear, and developing methods to enhance the efficiency of the catalysis is critical. Herein, we proposed a new strategy for flexibly modulating the hot electrons using a nonmetallic plasmonic heterostructure (named W₁₈O₄₉-nanowires/reduced-graphene-oxides) for isopropanol dehydration where the reaction rate was 180-fold greater than the corresponding thermocatalytic pathway. The key detail to this strategy lies in the synergetic utilization of ultraviolet light and visible-near-infrared light to enhance the hot electron generation and promote electron transfer for C-O bond cleavage during isopropanol dehydration reaction. This, in turn, results in a reduced reaction activation barrier down to 0.37 eV (compared to 1.0 eV of thermocatalysis) and a significantly improved conversion efficiency of 100% propylene from isopropanol. This work provides an additional strategy to modulate hot carrier of plasmonic semiconductors and helps guide the design of better catalytic materials and chemistries.

Dynamic Active Sites in Bi5O7I Promoted by Surface Tensile Strain Enable Selective Visible Light CO2 Photoreduction

Surface defects with abundant localized electrons on bismuth oxyhalide catalysts are proved to have the capability to capture and activate CO₂. However, bismuth oxyhalide materials are susceptible to photocorrosion, making the surface defects easily deactivated and therefore losing their function as active sites. Construction of deactivation-resistant surface defects on catalyst is essential for stable CO₂ photoreduction, but is a universal challenge. In this work, the Bi₅O₇I nanotubes with surface tensile strain are synthesized, which are favorable for the visible light-induced dynamic I defects generation. The CO₂ molecules absorbed on I defects are constantly reduced by the incoming photogenerated electrons from I-deficient Bi₅O₇I nanotubes and the successive protonation of CO₂ molecules is thus highly promoted, realizing the selective CO₂ conversion process via the route of CO₂-COOH^--CO. The efficient and stable photoreduction of CO₂ into CO with 100% selectivity can be achieved even under visible light (λ >420 nm) irradiation benefited from the dynamic I defects as active sites. The results presented herein demonstrate the unique action mechanism of light-induced dynamic defects during CO₂ photoreduction process and provide a new strategy into rational design of deactivation-resistant catalysts for selective CO₂ photoreduction.

Self-supporting photocatalyst of 2D Bi2O3 anchored on carbon paper for degradation pollutants

Two-dimensional vertically aligned Bi₂O₃ nanosheets over carbon paper (CP) were prepared via an in situ growth approach. Bi₂O₃/CP exhibits a robust photocatalytic activity, as well as renewability and flexibility. With Rhodamine B and 2,4-dichlorophenol used as target pollutants, the rate constant of Bi₂O₃/CP was 3.72 × 10⁻³ min⁻¹ and 6.93 × 10⁻³ min⁻¹ under visible-light irradiation for 2 h, respectively. The improved activity was attributed to the synergistic effects of the hierarchical structure of Bi₂O₃ and the conductive substrate, CP; the former provided efficient catalytic sites for the pollutants and absorbed more of the light scattered among the nanosheets, while the latter is beneficial to the photogenerated electron transfer.

Two-dimensional vertically aligned Bi₂O₃ nanosheets over carbon paper (CP) were prepared via an in situ growth approach.

Defect Engineering of Photocatalysts towards Elevated CO2 Reduction Performance

Photocatalytic CO₂ reduction provides a promising solution to address the crises of massive CO₂ emissions and fossil energy shortages. As one of the most effective strategies to promote CO₂ photoconversion, defect engineering shows great potential in modulating the electronic structure and light absorption properties of photocatalysts while increasing surface active sites for CO₂ activation and conversion. This Review summarizes the recent progress in defect engineering of photocatalysts to promote CO₂ reduction performances from the following four aspects: 1) Approaches to defect (mainly vacancy and dopant) generation in photocatalysts; 2) defect structure characterization techniques; 3) physical and chemical properties of defect-engineered photocatalysts; 4) CO₂ reduction performance enhancements in activity, selectivity, and stability of photocatalysts by defect engineering. This Review is expected to present readers with a comprehensive view of progress in the field of photocatalytic CO₂ reduction through defect engineering for elevated CO₂ -to-fuels conversion efficiency.

Ultrastable and Efficient Visible-light-driven CO2 Reduction Triggered by Regenerative Oxygen-Vacancies in Bi2 O2 CO3 Nanosheets

Herein, we first design a fast low-pressure ultraviolet light irradiation strategy for easily regenerating the nearly equivalent surface vacancies. Taking the defective Bi₂ O₂ CO₃ nanosheets as an example, nearly equal amount of oxygen vacancies can be regenerated under UV light irradiation. Synchrotron-radiation quasi in situ X-ray photoelectron spectra disclose the Bi sites in the O-defective Bi₂ O₂ CO₃ nanosheets can act as the highly active sites, which not only help to activate CO₂ molecules, but also contribute to stabilizing the rate-limiting COOH* intermediate. Also, in situ Fourier transform infrared spectroscopy and in situ mass spectrometry unravel the UV light irradiation contributes to accelerating CO desorption process. As a result, the O-defective Bi₂ O₂ CO₃ nanosheets achieve a stability up to 2640 h over 110 cycling tests and a high evolution rate of 275 μmol g^-1  h^-1 for visible-light-driven CO₂ reduction to CO.

Efficient Infrared-Light-Driven CO2 Reduction Over Ultrathin Metallic Ni-doped CoS2 Nanosheets

Converting CO₂ and H₂ O into carbon-based fuel by IR light is a tough task. Herein, compared with other single-component photocatalysts, the most efficient IR-light-driven CO₂ reduction is achieved by an element-doped ultrathin metallic photocatalyst-Ni-doped CoS₂ nanosheets (Ni-CoS₂ ). The evolution rate of CH₄ over Ni-CoS₂ is up to 101.8 μmol g^-1  h^-1 . The metallic and ultrathin nature endow Ni-CoS₂ with excellent IR light absorption ability. The PL spectra and Arrhenius plots indicate that Ni atoms could facilitate the separation of photogenerated carriers and the decrease of the activation energy. Moreover, in situ FTIR, DFT calculations, and CH₄ -TPD reveal that the doped Ni atoms in CoS₂ could effectively depress the formation energy of the *COOH, *CHO and desorption energy of CH₄ . This work manifests that element doping in atomic level is a powerful way to control the reaction intermediates, providing possibilities to realize high-efficiency IR-light-driven CO₂ reduction.